Here's a couple of sample answers:
The single scariest moment I have ever had in entertainment came during Diabolique [the 1955 film directed by Henri-Georges Clouzot]. It is the moment when the corpse in the bathtub opens its eyes and shows nothing but bulging whites. - Stephen King
Nothing. - Elvira
I've always loved ghost stories, writers like M.R. James, L.P. Hartley, Joan Aiken, Stephen King, Joe Hill. But the scariest story I've ever heard was a true ghost story.
There were eight or nine of us at a restaurant in Raleigh, North Carolina, and we were telling ghost stories. The friend of a friend said, 'When I was a girl living in Texas, I had a recurring dream. In this dream, I was walking down the street of my hometown, and a man would walk toward me. Sometimes he was older and sometimes he was younger. He didn't always have the same face, but I always knew it was the same man. He would get closer and closer, and I would know that something bad was going to happen, but I would wake up each time before he reached me. I would be terrified. One night, in my dream, we finally got face to face and I spoke to him. I said, "What is your name?" He said, "My name is Sammy." And then I woke up, and I was so afraid that I couldn't go back to sleep. I went to my sister's room and said, "Can I get in bed with you? I've just had a really bad dream." My sister said, "Was it Sammy?" I said, "What did you say? How do you know Sammy?" And my sister said, "I don't. But you just brought him in the room with you." I turned on the lights and I saw that my sister was asleep. - Kelly Link
Other respondents include Eli Roth, R. L. Stine, Guillermo del Toro, Frank Darabont, and Joe Hill.